Induction of two indirect defences benefits Lima bean (Phaseolus lunatus, Fabaceae) in nature

Abstract

Summary Herbivore damage often induces the release of herbivore‐induced plant volatiles (HIPVs) or the secretion of extrafloral nectar (EFN). The purpose of this study was to induce such proposed indirect defence responses in plants growing in their natural environment and to monitor the effects on vegetative and generative traits such as herbivory, plant growth and fruit set. I tested whether mature wild Lima beans (Phaseolus lunatus, Fabaceae) growing in Mexico could be reliably induced to produce HIPVs and EFN in response to application of the phytohormone, jasmonic acid (JA), and investigated whether this treatment leads to any benefit to the plants. Eleven HIPVs were released from JA‐treated leaves at significantly higher amounts than from control leaves, and only one (methylsalicylate) at significantly lower amounts than from controls. Treatment with JA increased EFN secretion on leaves on average by more than 30 times and on bracts by more than 20 times. EFN production, which has not previously been described in Lima bean, represents a second induced indirect resistance trait of this species. JA treatment reduced both the number of dead shoot tips and leaf damage by herbivores. Treated tendrils grew faster and produced more leaves than controls. At the end of the experiment, treated tendrils bore c. two times more inflorescences and three times more fruits than controls. Repeated treatment with JA resulted in a combined induction of HIPVs and EFN and strongly benefited wild Lima beans. The plants suffered less from the consequences of herbivore attack, supporting the use of the term ‘induced defences’ for these traits. JA‐induced defences thus can significantly benefit Lima bean under natural conditions.